The so-called "embedded servo" type magnetic disk apparatus employs such a disk as to have servo information written on the data area in order for the information to cause the magnetic head to follow a desired data track. The magnetic disk used in the apparatus of this type has a plurality of servo sectors for servo information and a plurality of data sectors for data alternately arranged on the data area (for example, see JP-B-55-20302).
In the conventional magnetic disk of this type, after servo information is recorded on the magnetic disk, the magnetic head is located with high fidelity at the center of a track based on the servo information, and it records and reproduces the data information on and from the track.
Also, in the magnetooptical disk apparatus, after the servo information is transferred to the magnetooptical disk by photolithography or the like, the magnetooptical head is located with high fidelity at the center of a track based on the servo information, and it records and reproduces the data information.
In the above conventional head-positioning method, however, if the centers of tracks at which the magnetic head or magnetooptical head is located on the basis of the servo information have initial eccentricities with respect to the rotation center of the disk, since the magnetic head or magnetooptical head tries to follow the track center lines with high fidelity and to record or reproduce data information, vibration will occur which synchronizes with the disk rotation while the head is being moved to follow the tracks. This vibration problem will be described with reference to FIG. 1(a).
FIG. 1(a) schematically shows the deviation e between a rotation center 11 of spindle for a magnetic disk 10 and a center of circles (pattern center) 12 which are formed by the center lines of tracks that are specified by the servo information recorded on the magnetic disk 10. As illustrated in FIG. 1(a), the rotation center 11 of spindle on the magnetic disk 10 is assumed to be deviated by e from the center 12 of circles of the servo pattern. Accordingly, the center line 13 of m-th data track and center line 14 of (m+1)-th data track specified by the servo pattern are decentered while the disk is being rotated. For example, the track center 13 is detected according to the servo pattern present within each of the n-th servo sector 15a, (n+1)-th servo sector 15b, (n+2)-th servo sector 15c and (n+3)-th servo sector 15d (the followings are omitted) of the m-th data track, so that a locus which the head 16 is required to trace can be found. In this case, when the movement of the head 16 is observed from the outside of the disk (, or when it is plotted on absolute coordinates), a sinusoidal undulation can be seen as shown in FIG. 1(b) at 19a. The amplitude of this undulation is represented by e. When the movement of the head 16 is observed from the disk 10 (, or when it is plotted on relative coordinates), however, a sinusoidal undulation can also be seen of which the amplitude is 0 if the head can perfectly follow the track center 13 or e/100 if the suppression ratio of the head servo system around the revolution rate is set for about -40 dB in the apparatus, as shown in FIG. 1(c). Therefore, since the head is positioned to be substantially zero in its relative position as shown in FIG. 1(c) at 19c, it vibrates on the absolute coordinates as illustrated in FIG. 1(b) at 19a.
The head system itself moves according to the above vibration, thus excites the mechanical system to vibrate, and finally decreases the positioning precision itself.
Thus, the conventional magnetic apparatus has the problem that when the rotation center of the disk is deviated from the pattern center, a vibration is caused in synchronism with the disk rotation.
Accordingly, it is an object of the invention to provide a magnetic recorder capable of preventing a vibration from being caused in synchronism with the rotation of the magnetic disk when the rotation center of the magnetic disk is deviated from the pattern center.
For the same reasons, the conventional magnetooptical disk recorder also has the problem that when the rotation center of the disk is deviated from the pattern center, a vibration is caused in synchronism with the disk rotation.
Accordingly, it is another object of the invention to provide a magnetooptical recorder capable of preventing a vibration from being caused in synchronism with the rotation of the magnetooptical disk when the rotation center of the magnetooptical disk is deviated from the pattern center.